Biomedical subjects

S Derycke

Publications and source records attributed to S Derycke.

10 recordsLinked to original sources

PubMed2000

Optimization of beam weights in conformal radiotherapy planning of stage III non-small cell lung cancer: effects on therapeutic ratio.

PURPOSE: To evaluate the effects of beam weight optimization for 3D conformal radiotherapy plans, with or without beam intensity modulation, in Stage III non-small cell lung cancer (NSCLC). METHODS AND MATERIALS: Ten patients with Stage III NSCLC were planned using a conventional 3D technique and a technique involving noncoplanar beam intensity modulation (BIM). Two planning target volumes (PTVs) were defined: PTV1 included macroscopic tumor volume and PTV2 included macroscopic and microscopic tumor volume. Virtual simulation defined the beam shapes and incidences as well as the wedge orientations (3D) and segment outlines (BIM). Weights of wedged beams, unwedged beams, and segments were determined by human trial and error for the 3D-plans (3D-manual), by a standard weight table (SWT) for the BIM-plans (BIM-SWT) and by optimization (3D-optimized and BIM-optimized) using an objective function with a biological and a physical component. The resulting non-optimized and optimized dose distributions were compared, using physical endpoints, after normalizing the median dose of PTV1 to 80 Gy. RESULTS: Optimization improved dose homogeneity at the target for 3D- and BIM-plans and the minimum dose at PTV1. The minimum dose at PTV2 was decreased by optimization especially in 3D-plans. After optimization, the dose-volume histograms (DVHs) of lung and heart were shifted to lower doses for 80-90% of the organ volume. Since lung is the dose-limiting organ in Stage III NSCLC, an increased minimum dose at PTV1 together with a decreased dose at the main lung volume suggests an improved therapeutic ratio. Optimization allows 10% dose escalation for 3D-plans and 20% for BIM-plans at isotoxicity levels of lung and spinal cord. Upon dose escalation, esophagus may become the dose-limiting structure when PTV1 extends close to the esophagus. CONCLUSIONS: Optimization using a biophysical objective function allowed an increase of the therapeutic ratio of radiotherapy planning for Stage III NSCLC.

Algorithms↗

PubMed2000

Validation of MR-based polymer gel dosimetry as a preclinical three-dimensional verification tool in conformal radiotherapy.

The aim of this work was to investigate MR-based polymer gel dosimetry as a three-dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water-filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin-spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root-mean-square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized.

Acrylamide↗

PubMed1999

Inhomogeneous target-dose distributions: a dimension more for optimization?

PURPOSE: To evaluate if the use of inhomogeneous target-dose distributions, obtained by 3D conformal radiotherapy plans with or without beam intensity modulation, offers the possibility to decrease indices of toxicity to normal tissues and/or increase indices of tumor control stage III non-small cell lung cancer (NSCLC). METHODS AND MATERIALS: Ten patients with stage III NSCLC were planned using a conventional 3D technique and a technique involving noncoplanar beam intensity modulation (BIM). Two planning target volumes (PTVs) were defined: PTV1 included macroscopic tumor volume and PTV2 included macroscopic and microscopic tumor volume. Virtual simulation defined the beam shapes and incidences as well as the wedge orientations (3D) and segment outlines (BIM). Weights of wedged beams, unwedged beams, and segments were determined by optimization using an objective function with a biological and a physical component. The biological component included tumor control probability (TCP) for PTV1 (TCP1), PTV2 (TCP2), and normal tissue complication probability (NTCP) for lung, spinal cord, and heart. The physical component included the maximum and minimum dose as well as the standard deviation of the dose at PTV1. The most inhomogeneous target-dose distributions were obtained by using only the biological component of the objective function (biological optimization). By enabling the physical component in addition to the biological component, PTV1 inhomogeneity was reduced (biophysical optimization). As indices for toxicity to normal tissues, NTCP-values as well as maximum doses or dose levels to relevant fractions of the organ's volume were used. As indices for tumor control, TCP-values as well as minimum doses to the PTVs were used. RESULTS: When optimization was performed with the biophysical as compared to the biological objective function, the PTV1 inhomogeneity decreased from 13 (8-23)% to 4 (2-9)% for the 3D-(p = 0.00009) and from 44 (33-56)% to 20 (9-34)% for the BIM plans (p < 0. 00001). Minimum PTV1 doses (expressed as the lowest voxel-dose) were similar for both objective functions. The mean and maximum target doses were significantly higher with biological optimization for 3D as well as for BIM (all p values < 0.001). Tumor control probability (estimated by TCP1 x TCP2) was 4.7% (3D) and 6.2% (BIM) higher for biological optimization (p = 0.01 and p = 0.00002 respectively). NTCP(lung) as well as the percentage of lung volume exceeding 20 Gy was higher with the use of the biophysical objective function. NTCP(heart) was also higher with the use of the biophysical objective function. The percentage of heart volume exceeding 40 Gy tended to be higher but the difference was not significant. For spinal cord, the maximum dose as well as NTCP(cord) were similar for 3D plans (D(max): p = 0.04; NTCP: p = 0.2) but were significantly lower for BIM (D(max): p = 0.002; NTCP: p = 0.008) if the biophysical objective function was used. CONCLUSIONS: When using conventional 3D techniques, inhomogeneous dose distributions offer the potential to further increase the probability of uncomplicated local control. When using techniques as BIM that would lead to large escalation of the median and maximum target doses, it seems indicated to limit target-dose inhomogeneity to avoid dose levels that are so high that the safety becomes questionable.

Carcinoma, Non-Small-Cell Lung↗

PubMed1999

Clinical delivery of intensity modulated conformal radiotherapy for relapsed or second-primary head and neck cancer using a multileaf collimator with dynamic control.

BACKGROUND AND PURPOSE: Concave dose distributions generated by intensity modulated radiotherapy (IMRT) were applied to re-irradiate three patients with pharyngeal cancer. PATIENTS, MATERIALS AND METHODS: Conventional radiotherapy for oropharyngeal (patients 1 and 3) or nasopharyngeal (patient 2) cancers was followed by relapsing or new tumors in the nasopharynx (patients 1 and 2) and hypopharynx (patient 3). Six non-opposed coplanar intensity modulated beams were generated by combining non-modulated beamparts with intensities (weights) obtained by minimizing a biophysical objective function. Beamparts were delivered by a dynamic MLC (Elekta Oncology Systems, Crawley, UK) forced in step and shoot mode. RESULTS AND CONCLUSIONS: Median PTV-doses (and ranges) for the three patients were 73 (65-78), 67 (59-72) and 63 (48-68) Gy. Maximum point doses to brain stem and spinal cord were, respectively, 67 Gy (60% of volume below 30 Gy) and 32 Gy (97% below 10 Gy) for patient 1; 60 Gy (69% below 30 Gy) and 34 Gy (92% below 10 Gy) for patient 2 and 21 Gy (96% below 10 Gy) at spinal cord for patient 3. Maximum point doses to the mandible were 69 Gy for patient 1 and 64 Gy for patient 2 with, respectively, 66 and 92% of the volume below 20 Gy. A treatment session, using the dynamic MLC, was finished within a 15-min time slot.

Adult↗

PubMed1998

Conformal radiotherapy of Stage III non-small cell lung cancer: a class solution involving non-coplanar intensity-modulated beams.

PURPOSE: We developed a semiautomatic class solution to irradiate centrally located Stage III non-small cell lung cancer (NSCLC), involving a beam intensity modulation technique and optimization using a biophysical cost function. METHODS AND MATERIALS: Treatment for 10 patients with Stage III NSCLC was planned, using a conventional three- or four-beam three-dimensional (3D) technique and two techniques involving, respectively, seven (BIM1) and five (BIM2) noncoplanar beam incidences with intensity modulation. Two planning target volumes were defined: PTV1 included macroscopic tumor volume and PTV2 included macroscopic and microscopic disease. Beams were divided into beam parts (segments) and their outlines were defined during virtual simulation. Optimization using a biophysical cost function determined beam weights, segment weights, and wedge angles. Biological end points included tumor control probability of both target volumes (TCP1 and TCP2) and normal tissue complication probability (NTCP) of heart, lung, and spinal cord. The resulting uncomplicated local control probability (UCLP) was calculated. Physical end points included dose at PTV1 expressed as a dose minimum and dose maximum. Target-dose inhomogeneity was constrained in all plans. RESULTS: Concerning tumor evaluation, TCP1 was 74% (range 54-89%) for the 3D plan, 78.0% (range 62-94%) for BIM1, and 86.0% (range 59-93%) for BIM2. TCP1*TCP2 was, respectively, 67.0% (range 39-81%), 73.0% (range 56-94%), and 81.0% (range 54-93%). Minimum doses to PTV1 were 85, 80, and 88 Gy with the three respective techniques, while dose maxima were 89, 101, and 100 Gy. NTCPs of lung were 45.0% (range 11-75%) for 3D, 19.5% (range 8-59%) for BIM1, and 24.5% (range 3-61%) for BIM2. NTCPs of heart and spinal cord were comparable for all techniques. ULCPs were 37.0% (range 9-73%), 52.5% (range 22-86%), and 60.0% (range 20-85%), respectively. Applying physical limits to ensure clinical safety, minimum doses at PTV1 were recalculated. These were 72, 71, and 74 Gy for 3D, BIM1, and BIM2, respectively. CONCLUSION: The BIM2 plan is a candidate class solution for dose escalation studies in centrally located Stage III NSCLC.

Algorithms↗

PubMed1998

Three-dimensional dosimetry using polymer gel and magnetic resonance imaging applied to the verification of conformal radiation therapy in head-and-neck cancer.

BACKGROUND AND PURPOSE: It was our aim to investigate NMR-based BANG gel dosimetry as a three-dimensional dosimetry technique in conformal radiotherapy. MATERIALS AND METHODS: The BANG gel consisting of gelatin, water and co-monomers was first validated in a cylindrical glass flask for a single standard beam. Next, the gel contained in a human neck-shaped cast was used to verify a treatment plan for the conformal irradiation of a concave tumour in the lower neck. Magnetic resonance relaxation rate images were acquired and, based on an appropriate calibration of the gel, converted to absorbed dose distributions. The resulting maps were compared with dose distributions measured using radiographic film. RESULTS: The gel-measured dose profiles of standard beams agreed within 3% (root mean square difference) with the profiles measured with high spatial resolution by a diamond detector. For the multi-beam conformal treatment, the difference map between gel-measured and film-measured dose distributions revealed a noise component and a more systematic deviation including structural or space-coherent patterns. The mean absolute value of the difference amounted to 8%. A number of possible causes for this deviation are designated. CONCLUSIONS: Polymer gel dosimetry in combination with magnetic resonance imaging is a promising method for dosimetric verification of conformal radiotherapy.

Dose-Response Relationship, Radiation↗

PubMed1997

Non-coplanar beam intensity modulation allows large dose escalation in stage III lung cancer.

PURPOSE: To evaluate the feasibility of dose escalation in stage III non-small cell lung cancer, we compared standard coplanar (2D) with non-coplanar beam arrangements, without (3D) and with beam intensity modulation (3D-BIM). MATERIALS AND METHODS: This study was a planning effort performed on a non-selected group of 10 patients. Starting from a serial CT scan, treatment planning was performed using Sherouse's GRATIS 3D planning system. Two target volumes were defined; gross tumor volume (GTV) defined a high-dose target volume that had to receive a dose of at least 80 Gy and GTV plus the lymph node regions with >10% probability of invasion defined an intermediate-dose target volume (GTV + N). It was our intention to irradiate GTV + N up to 56 Gy or more. If the prescribed doses on GTV and GTV + N could not be reached with either the 2D or 3D technique, a 3D-BIM plan was performed. The 3D-BIM plan was a class solution involving identical gantry angles, segment arrangements and relative segment weights for all patients. Dose volume histograms for GTV, GTV + N, lung and spinal cord were calculated. Criteria for tolerance were met if no points inside the spinal cord exceeded 50 Gy and if at least 50% of the lung volume received less than 20 Gy. Under these constraints, maximal achievable doses to GTV and GTV + N were calculated. RESULTS: In all 2D plans, spinal cord was the limiting factor and the prescribed doses for GTV and GTV + N could not be reached in any patient. The non-coplanar 3D plan resulted in a satisfying solution in 4 out of 10 patients under the same constraints. In comparison with 2D, the minimum dose in GTV + N was increased. Six patients had to be planned with the 3D-BIM technique. The theoretical minimum dose to GTV + N ranged between 56 and 98 Gy. The delivery of 80 Gy or more to GTV was possible in all patients. For a minimal dose of 80 Gy to GTV, the maximal dose to any point of the spinal cord varied between 27 and 46 Gy. The lung volume receiving more than 20 Gy ranged from 26 to 46%. CONCLUSION: The potential of 3D-BIM for dose escalation is explained as follows: (i) compared to other planning techniques, a larger amount of lung tissue can be spared by using beam directions that are well-aligned with the mediastinal structures. Such beam directions have narrow angles with the sagittal plane; (ii) dividing all beams into segments with well-specified geometrical restrictions in relation to the spinal cord and well-defined relative weights results in a lower dose to the spinal cord.

Carcinoma, Non-Small-Cell Lung↗

PubMed1996

Planning and delivering high doses to targets surrounding the spinal cord at the lower neck and upper mediastinal levels: static beam-segmentation technique executed with a multileaf collimator.

BACKGROUND AND PURPOSE: It remains a technical challenge to limit the dose to the spinal cord below tolerance if, in head and neck or thyroid cancer, the planning target volume reaches to a level below the shoulders. In order to avoid these dose limitations, we developed a standard plan involving Beam Intensity Modulation (BIM) executed by a static technique of beam segmentation. In this standard plan, many machine parameters (gantry angles, couch position, relative beam and segment weights) as well as the beam segmentation rules were identical for all patients. MATERIALS AND METHODS: The standard plan involved: the use of static beams with a single isocenter; BIM by field segmentation executable with a standard Philips multileaf collimator; virtual simulation and dose computation on a general 3D-planning system (Sherouse's GRATIS); heuristic computation of segment intensities and optimization (improving the dose distribution and reducing the execution time) by human intelligence. The standard plan used 20 segments spread over 8 gantry angles plus 2 non-segmented wedged beams (2 gantry angles). RESULTS: The dose that could be achieved at the lowest target voxel, without exceeding tolerance of the spinal cord (50 Gy at highest voxel) was 70-80 Gy. The in-target 3D dose-inhomogeneity was approximately 25%. The shortest time of execution of a treatment (22 segments) on a patient (unpublished) was 25 min. CONCLUSIONS: A heuristic model has been developed and investigated to obtain a 3D concave dose distribution applicable to irradiate targets in the lower neck and upper mediastinal regions. The technique spares efficiently the spinal cord and allows the delivery of higher target doses than with conventional techniques. It can be planned as a standard plan using conventional 3D-planning technology. The routine clinical implementation is performed with commercially available equipment, however, at the expense of extended execution times.

Dose-Response Relationship, Radiation↗

PubMedDate not supplied

Seasonal dynamics of population genetic structure in cryptic taxa of the Pellioditis marina complex (Nematoda: Rhabditida).

The distribution patterns and genetic structure of the Pellioditis marina species complex in Belgium and The Netherlands were compared between four consecutive seasons. Different types of habitats (coast, estuary, semi-estuary and lake) with different degrees of connectivity were sampled. In addition, each habitat type was characterised by either temporal or permanent algal deposits. We screened 426 bp of the mitochondrial cytochrome oxidase c (COI) gene with the single-strand conformation polymorphism (SSCP) method in 1615 individuals of Pellioditis marina. The 51 haplotypes were divided into four (sympatric) lineages, with divergences ranging from 0.25 to 10.6%. Our results show that the lineages have different temporal dynamics, which may be linked to abiotic factors. Analysis of Molecular Variance (AMOVA) indicated a significant structuring in the PmI lineage, which correlated with habitat characteristics and which changed over time (Mantel, r = 0.51; p = 0.126). Intrapopulational diversity was similar in all locations, and temporal changes in haplotype frequencies were not higher in temporary than in permanent algal deposits. Instead, the results of the temporal survey indicated that (some) P. marina populations are characterised by a metapopulation structure. It is emphasized that a complete and correct interpretation of processes causing genetic structuring within species and of the genetic structure itself can only be done when analyses are performed at several time points.

Animals↗

PubMedDate not supplied

[Intensity modulated radiotherapy with dynamic multileaf collimator. Technique and clinical experience].

Since early 1997, dynamic multileaf collimators (DMLCs) have been used in our division for intensity modulated radiotherapy (IMRT). We have used IMRT to: irradiate concave targets (head and neck, paraspinal tumors); combine beams with shallow hinge angles (mediastinum, lung tumors); deliver intentionally inhomogeneous dose distributions (prostate, paranasal sinuses, brain tumors). IMRT is now our standard treatment for locoregional relapse (after high-dose radiotherapy) for head and neck cancer and for radical treatment of localized prostate cancer. For a variety of other tumors, conventional 3D-plans are compared with IMRT-plans, the latter being clinically implemented if superior. We developed a geometry based IMRT planning strategy to create assemblies of static intensity modulated (IM)-beams which consist of uniform (unmodulated) segments. By a translator program, segments are combined in a single prescription which allows delivery under computer control. Cost-containment is further improved by automation of the planning. After manual or semi-automated contouring of PTV and the organs at risk, prostate IMRT plans, based on a class solution, are generated and optimized by a computer. IMRT for pharyngeal relapses and most other tumor sites is planned semi-automatically. IMRT replaces gradually conventional treatments in our division. Interesting dose distributions generated by IMRT allow a better sparing of normal tissues with decreased acute and late toxicity, and offer a window for further dose escalation.

Algorithms↗